This invention relates generally to techniques for the identification and analysis of contaminant particles on semiconductor wafers and, more particularly, to techniques for locating particles more reliably when using a high-magnification imaging device, such as a scanning electron microscope. Semiconductor fabrication technology today deals with wafer sizes up to 200 mm (millimeters) and feature geometries with dimensions well below 1 .mu.m (micrometer). The presence of a particle larger than about a tenth the width of a conductive line on a wafer can lead to failure of a semiconductor chip made from the wafer. Therefore, a critical task facing semiconductor process engineers is to identify and, as far as possible, to eliminate sources of surface contamination.
A well known approach to identification and analysis of contaminant particles involves the use of a scanning electron microscope (SEM) to locate particles on an unpatterned semiconductor wafer, referred to as a mirror wafer. Scanning X-ray spectroscopy or optical microscopy systems may also be used for this purpose. Because critical particles sizes may be as small as 0.1-0.2 .mu.m, the SEM must use a magnification of approximately 2000.times. to allow the smallest critical particles to be seen by the human eye, which has a resolution limitation of approximately 0.2 mm. At this high level of magnification, it is extremely difficult to find a particle on the wafer unless its position is known quite accurately before the SEM or similar system is used.
Because the SEM must be operated only at high levels of magnification, it is not a useful instrument to obtain an overview of particles on an entire wafer. Other devices have been developed for this purpose and a two-stage process for locating and analyzing particles is normally employed. In the first stage, the wafer is raster-scanned with a laser beam to locate practically all of the particles on the wafer or in an area of interest on the wafer. The manner in which the laser beam is scattered from the particles yields signals from which the approximate sizes and locations of the particles can be deduced, for output to a computer display screen. However, because the scattering mechanism is not completely understood, the signals are of little help in identifying the type, chemical composition, and possible source of contaminant particles. This information can only be obtained with the help of a high-magnification imaging device, such as an SEM.
As already mentioned, locating a particle using an SEM can be extremely difficult or impossible unless the position of the particle is first known to some degree of accuracy. Specifically, at a magnification of 2000.times. a 0.1 .mu.m particle must be "targeted" to an accuracy of about 40 .mu.m in order to be located on the SEM viewing screen. Basically, the laser scanning technique provides estimated particle positions, in terms of x and y coordinates. To locate a particle of interest with the SEM, the estimated coordinates for that particle are fed into the SEM, which, at high magnification, provides a view of only a very small part of the wafer. The targeting error is the distance between the estimated position of the particle and its actual position when viewed on the SEM viewing screen. If the targeting error is greater than about 40 .mu.m, the particle will not even appear on the viewing screen and an extended search of the viewing screen may be required. In some cases, the particle may never be located in the SEM.
A critical aspect of this two-stage particle analysis method is that the coordinate system used in the laser scanning device must be transformed to the coordinate system used in the SEM or other similar device. In most systems, the wafer has to be physically moved from one device to the other and, in any event, the coordinate systems used in the laser scanning device and the SEM will be totally different. When a wafer is first placed in the SEM, the wafer edges and a reference feature (a notch or a flat segment in the edge) are detected by the SEM and used to provide a first approximation for transformation of the coordinates from the scanning device to the SEM. An improved transformation is then obtained by identifying, in both devices, two reference particles that are relatively recognizable, because of their size and contrast. Given the coordinates of these two reference particles, as measured in the coordinate systems of both devices, a simple and well known coordinate transformation can be used to transform the other particle locations from one coordinate system to the other. Each particle may then be more easily located in the SEM and further analyzed to determine its characteristics and possible source.
The two-stage particle location and analysis technique briefly described above has been used with some measure of success, but still has at least one significant disadvantage. It has been found that the accuracy with which the particles can be located depends in part on the positions of the particles selected as reference particles. Specifically, the targeting error may vary between 50.mu.m and 300.mu.m, depending on particle position on the wafer. As a result of this, nearly one-third of the particles may not be found using the SEM.
The cross-referenced application describes and claims several related techniques for improving the targeting accuracy in the two-stage process. One of these techniques involves subjecting the wafer to one or more additional scans during the first stage of processing, in the laser scanning device. At the time the cross-referenced application was filed, this double-scan method was not fully understood. Some scanning devices incorporate automatic averaging of particle coordinate data from two consecutive scans, but the averaged data do not provide improvement over a single scan. An improved multiple-scan method has subsequently been invented, yielding further improvement in targeting accuracy, and is described below.